1. Field of the Invention
The present invention relates to a receiving device that receives a signal transmitted from a driver that transmits a small-amplitude differential signal, a driving unit having the receiving device, and an image forming apparatus having the driving unit.
2. Description of the Related Art
In recent years and continuing to the present, attention has been paid to a LVDS (Low Voltage Differential Signal) method as a data transmission method for fulfilling demands for high-speed data. The LVDS method is used in PPCs (Plain Paper Copiers) or laser printers, for example, when an image processing processor transmits data to a laser driver that drives laser diodes.
A general LVDS method is implemented in such a manner that a LVDS driver and a LVDS receiver are connected to each other via a connection cable. If input data of the input terminal of the LVDS receiver become indefinite in the general LVDS method, output data of the LVDS receiver also become indefinite.
If output data of the LVDS receiver become indefinite when the LVDS method is used for transmitting data between the image processing processor and the laser driver described above, there arise problems, for example, in controlling the light emission of the laser diodes.
In order to deal with such problems, known LVDS receivers are configured to detect the indefinite status of input data applied to an input terminal. FIGS. 1A and 1B are diagrams showing examples of the known LVDS receivers, and FIG. 2 is a diagram showing another example of the known LVDS receiver.
For example, if input terminals A and B are broken in the LVDS receivers shown in FIGS. 1A and 1B, pull-up resistors Rp make the voltages of wiring connected to the input terminals A and B close to a power supply voltage. At this time, the LVDS receivers shown in FIGS. 1A and 1B detect the broken statuses of the input terminals A and B. The LVDS receiver shown in FIG. 2 operates in the same manner as the LVDS receivers shown in FIGS. 1A and 1B.
Besides the LVDS receivers shown in FIGS. 1A, 1B, and 2, Patent Document 1 discloses a technology that transmits data using a low voltage differential signal and prevents, based on the data, an accident caused when laser beams are emitted.
With the above technology, however, an abnormality in a signal line cannot be detected if only one of the two input terminals (inverting input terminal and non-inverting input terminal) is broken. This is because both of the voltage of the inverting input terminal and that of the non-inverting input terminal of the LVDS receiver have the same potential due to a terminating resistor when only one of the input terminals is broken.
This phenomenon occurs with a reference voltage that determines an in-phase input voltage of the LVDS receiver because the resistance of the pull-up resistor is generally significantly greater than the terminating resistor.
Furthermore, in a case where wiring on the side of one of the input terminals is short-circuited (ground or power supply voltage), the voltage of the inverting input terminal and that of the non-inverting input terminal of the LVDS receiver may also have the same potential at an unintended voltage level (ground or power supply voltage). Accordingly, the abnormality in the signal line cannot be detected. Moreover, in a case where the inverting input terminal and the non-inverting input terminal are short-circuited, the voltage of the inverting input terminal and that of the non-inverting input terminal may also have the same potential. Accordingly, the abnormality in the signal line cannot be detected.
Furthermore, in the known LVDS receiver, a reference voltage that determines an in-phase input voltage of the LVDS receiver and output data on the side of the LVDS driver may be separately controlled. In this case, if an output on the side of the LVDS driver shows high impedance and the reference voltage is normally output, the voltages of the two input terminals of the LVDS receiver have the same potential. Accordingly, the abnormality in the signal line cannot be detected.
If the voltage of the inverting input terminal and that of the non-inverting input terminal have the same potential, output data of the LVDS receiver become indefinite.
If output data become indefinite, data cannot be properly transmitted from the LVDS receiver to a laser driver. In case that a load driven by the data transmitted from the LVDS receiver may have an affect on the human body as in the case of laser diodes, the abnormality in the signal line must be particularly detected to stop an output as soon as possible.
Furthermore, the LVDS driver and the LVDS receiver may have a different power supply, and the power supply of the LVDS driver and the withstand voltage of output transistors included in the LVDS driver may be small. If an output of the LVDS driver shows high impedance in this case, the power supply of the LVDS is directly applied to the LVDS driver, which may result in the LVDS driver being broken.
Patent Document 1: JP-B2-3798571